Traditionally, the cellular radio spectrum was partitioned by frequency as a Frequency Division Multiple Access (FDMA) system. The set of available cellular carrier frequencies was distributed among the various sites that make up the repeating reuse pattern that is characteristic of the stereotypical cellular honeycomb pattern. The frequencies were distributed in such a way so as to minimize interference from adjacent channels, and from co-channel uses of the same frequency at the reuse distance. This process of minimizing adjacent and co-channel interference is called cellular frequency planning. A variety of techniques have been developed for avoiding this frequency planning and improving capacity and trunking efficiency by striving for a "one-site repeat"--namely, that all frequencies are capable of being used non-interferingly at each and every cell site. For them to be used non-interferingly, various techniques must be employed, involving intra-cell space/time/frequency coordination and probabilistic reduction of intercellular interference.
As higher capacities were demanded of cellular systems, each FDMA radio carrier was subdivided into Time Division Multiple Access (TDMA) "time slots" in a repeating frame of slots where several different subscribers use the same carrier frequency time-multiplexed to carry their compressed, digitized speech. Ordinarily, a subscriber is allocated a certain repetitive time slot on a single carrier frequency for the duration of his call.
Since these TDMA systems are so well organized in the time domain, they provided opportunities for coordination of the frequency and space domains such that "single site repetition" can be realized. Certain of these TDMA system designs contemplate frequency hopping, but for purposes of noise and interference reduction, not necessarily for achieving one-site repetition and avoidance of frequency planning. Rather than the subscriber being allocated a designated time slot per frame on a single carrier frequency, he is allocated a designated time slot and a frequency hopping sequence. The allocation of these sequences constitutes a "code", thus comprising a subset of the field of Code Division Multiple Access (CDMA) systems. To ease the burden on frequency synthesizers in subscriber equipment, these sequences typically "walk" the synthesizer sequentially through the available radio spectrum. All subscribers typically have a staggered start on this walk through the frequencies so as to be non-interfering or "orthogonal"--such that no two transmissions are on the same frequency at the same time in the same cell. If these hopping sequences are co-ordinated (spatially) with surrounding cells, the necessary coordination will have been realized in space, time, and frequency. Thus, all of the frequencies can theoretically be used in every cell, achieving a "single site repeat".
However, a problem arises from the phased introduction of these FD/TD/CDMA systems. Typically, there is initially very limited radio spectrum available for any new systems; and that meager spectrum that is allocated is typically apportioned among various competing operators in one locale. Thus, the opportunities for managing interference in either frequency, time, or space is severely limited.
This invention takes as its object to overcome these shortcomings and to realize certain advantages presented below.